An Observational Study of Diurnal Variations of Marine Stratiform Cloud

Marine stratiform clouds are important and highly variable contributors to earth's radiation budget over the eastern subtropical oceans and over middle- to high-latitude oceans in summer. Because these clouds influence the radiation budget primarily through their albedo, their diurnal cycle has an important influence on their radiative effectiveness. The authors have analyzed the diurnal cycle in marine low-cloud fraction inferred from the International Satellite Cloud Climatology Project (ISCCP) dataset, after correcting for overlying clouds by using the assumption of random cloud overlap. The results have been compared with the diurnal cycle of low clouds at fixed ships and ships of opportunity. The diurnal cycles of ISCCP low clouds are in good agreement with surface observations of the diurnal cycle of low stratiform clouds almost everywhere. Analysis of the ISCCP data on a 2.5° × 2.5° grid shows that the largest diurnal range in low-cloud fraction occurs downwind in the mean flow, or westward and equatorward, of the subtropical maxima in low-cloud fraction. This is qualitatively consistent with control of the cloud amount by two competing processes in a partially decoupled cloud-topped planetary boundary layer: heating by solar radiation absorption and advection of moist boundary layer air. A radiative transfer code has been used to show that in eastern subtropical ocean regions, where the diurnal cycle of low clouds is large and the cloud has a small optical thickness, calculations with diurnally averaged cloud fraction overestimate total cloud radiative forcing by up to 3 W m−2 (16%) at the surface and 3 W m−2 (7%) at the top of the atmosphere compared with calculations that account for the diurnal cycle.